Ketene‐forming elimination reactions from aryl phenylacetates promoted by R 2 NH in MeCN: effects of base‐solvent and β ‐phenyl group

Elimination reactions of C 6 H 5 C(R)HCO 2 C 6 H 3 ‐2‐X‐4‐NO 2 [R = H ( 1 ), Ph ( 2 ), X = H ( a ), Cl ( b ), NO 2 ( c )] promoted by R 2 NH in MeCN have been studied kinetically. The reactions are second‐order and exhibit Brönsted β  = 0.46–0.89 and | β lg | = 0.37–0.76 and an E2 mechanism is evident. When the base‐solvent was changed from R 2 NH/R 2 NH   2 + –70 mol% MeCN(aq) to R 2 NH‐MeCN, β and | β lg | values remained nearly the same within experimental error. For eliminations from 1 and 2 , β and | β lg | values were nearly identical, although the rate was retarded by the β ‐Ph group. Noteworthy is the relative insensitivity of the ketene‐forming transition state to the base‐solvent and β ‐R group variation. Copyright © 2007 John Wiley & Sons, Ltd.